1

A. Teleki, M.K. Akhtar and S.E. Pratsinis“The quality of SiO2-coatings on flame-made TiO2-based nanoparticles”

Journal of Materials Chemistry Issue 30, 2008

06.07.2008

Flame Spray Synthesis and in-situCoating of Nanoparticles in One Step

Alexandra Teleki, Martin C. Heine, F. Krumeich, M.K. Akhtar and S.E. Pratsinis

Particle Technology Laboratory, ETH Zurich, Switzerland

3

Applications of SiO2-coated TiO2

UV-absorption in liquid suspensions[1] or polymer composites[2]

[1] Lademann, J., H. J. Weigmann, H. Schafer, G. Muller, and W. Sterry, Skin Pharmacol. Appl. Skin Physiol. 13, 258 (2000).[2] Nussbaumer, R. J., W. R. Caseri, P. Smith, and T. Tervoort, Macromol. Mater. Eng. 288, 44 (2003).

Rutile TiO2 in PS[3] SiO2-coated TiO2

Photocatalytic activity of TiO2 degrades surrounding matrixhigh rutile content and/or coatings

TiO2SiO2

[3] Chandra, A., L.-S. Turng, S. Gong, D.C. Hall, D.F. Caulfield, and H. Yang, Polymer Composites, 241 (2007).

4

Vapor-fed flame synthesis (chloride process) of pigmentary TiO2: ~ 2 million tons/year[1]

[1] Fisher, J., and T.A. Egerton, Titanium compounds, Inorganic, In Kirk-Othmer Encyclopedia of Chemical Technology, John Wiley & Sons, Inc., (2001).

In-situ SiO2-coating of flame-made TiO2

5

Si, Al and Ti

co-oxidized

amorphous

crystalline

co-oxidized SiO2/Al2O3/TiO2

Segregation in co-oxidized SiO2/Al2O3/TiO2

6

co-oxidized

amorphous

crystalline

Al-doped TiO2Vapor flame-made SiO2/TiO2[1]

Co-oxidation of Si/Ti precursors leads to segregation and separate SiO2 and TiO2 particles

[1] Teleki A., S.E. Pratsinis, R. Jossen, and F. Krumeich, J. Mater. Res. 20, 1336 (2005).

Segregation in co-oxidized SiO2/Al2O3/TiO2

co-oxidized SiO2/Al2O3/TiO2

7

Experimental set-up for in-situ coating

N2

Si prec.

Si injection

point

1. Si injection point[1]

(burner-ring-distance: BRD) (20 wt% SiO2, 15 l/min N2)

Si injection

point

25 g/h: 4 wt% Al2O3/TiO25 ml/min precursor solution (1 M Al(s-BuO)3/TTIP)5 l/min O2 dispersion40 l/min O2 sheath

Al and Ti

2. SiO2 content[2]

(15 l/min, 20 cm BRD)

[1] Lee, B. S., D.J. Kang, and S.G. Kim, J. Mater. Sci. 38, 3545 (2003).[2] Siddiquey, I.A., T. Furusawa, M. Sato, K. Honda, and N. Suzuki, Dyes and Pigments, 76, 754 (2008).

3. Co-oxidized SiO2/Al2O3/TiO2(15 l/min, 20 cm BRD)

Si, Al and Ti

N2

Si prec.

8

Si injection point, cm

0 5 10 15 20 25 30

μg A

ceto

ne/m

l IPA

0

50

100

150

200

250

300

Effect of Si Injection PointPhotooxidation of isopropanol (IPA) to acetone

Al/TiO2

9

Si injection point, cm

0 5 10 15 20 25 30

μg A

ceto

ne/m

l IPA

0

50

100

150

200

250

300

Effect of Si Injection Point

Si injection point, cm

0 5 10 15 20 25 30

μg A

ceto

ne/m

l IPA

0

50

100

150

200

250

300

Si injection point, cm

0 5 10 15 20 25 30

μg A

ceto

ne/m

l IPA

0

50

100

150

200

250

300

Photooxidation of isopropanol (IPA) to acetone

Al/TiO2

10

Si injection point, cm

0 5 10 15 20 25 30

μg A

ceto

ne/m

l IPA

0

50

100

150

200

250

300

Effect of Si Injection Point

Si injection point, cm

0 5 10 15 20 25 30

μg A

ceto

ne/m

l IPA

0

50

100

150

200

250

300

Si injection point, cm

0 5 10 15 20 25 30

μg A

ceto

ne/m

l IPA

0

50

100

150

200

250

300 Impact of Si on TiO2crystallinity and size[1]

is reduced at higher Si precursor injection points.

[1] Akhtar, M.K., S.E. Pratsinis, and S.V.R. Mastrangelo, J. Am. Ceram. Soc., 1992, 75, 3408.

pure TiO2

Si injection point, cm

0 5 10 15 20 25 30

μ g A

ceto

ne/m

l IPA

0

50

100

150

200

250

300

rutil

e w

t%

30

40

50

60

70

Photooxidation of isopropanol (IPA) to acetone

Al/TiO2

11

Si injection point, cm

0 5 10 15 20 25 30

μg A

ceto

ne/m

l IPA

0

50

100

150

200

250

300

Effect of Si Injection Point

Si injection point, cm

0 5 10 15 20 25 30

μg A

ceto

ne/m

l IPA

0

50

100

150

200

250

300

Si injection point, cm

0 5 10 15 20 25 30

μg A

ceto

ne/m

l IPA

0

50

100

150

200

250

300

Photooxidation of isopropanol (IPA) to acetone

20 cm

5 cm

10 cm

amorphous

crystalline

amorphous

crystalline

crystalline

amorphous

Premature Si injection results in separate SiO2and poorly-coated TiO2.

Al/TiO2

12

SiO2 wt% fraction

0 5 10 15 20

μg A

ceto

ne/m

l IPA

0

50

100

150

200

250

300

SiO2 wt% fraction

0 5 10 15 20

μg A

ceto

ne/m

l IPA

0

50

100

150

200

250

300

SiO2 wt% fraction

0 5 10 15 20

μg A

ceto

ne/m

l IPA

0

50

100

150

200

250

300

Effect of Si Content on Photocatalytic Activity

coating processco-oxidized

13

SiO2 wt% fraction

0 5 10 15 20

μg A

ceto

ne/m

l IPA

0

50

100

150

200

250

300 Co-oxidized: the rutilefraction decreases as SiO2 is added.

In the coating processthe rutile fraction israther independent of SiO2 content.

68 wt% rutile 36 wt% rutile

60 wt% rutile

coating processco-oxidized

Effect of Si Content on Photocatalytic Activity

14

SiO2 wt% fraction

0 5 10 15 20

μg A

ceto

ne/m

l IPA

0

50

100

150

200

250

300

amorphous

crystalline

20 wt% SiO2

co-oxidized

amorphous

crystalline

5 wt% SiO2

coating processco-oxidized

Effect of Si Content on Photocatalytic Activity

Theoretical coating thickness 5 wt% SiO2: < 1 nm 20 wt% SiO2: 2.7 nm

Increasing coating thickness

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Homogeneous, Smooth Coatings –No separate SiO2

Ti

Ti

Ti

20 wt% SiO2

16

Characterization of the SiO2 Coating Quality

Fast and quantitative characterization of coating quality

TEM: difficult to gather statistically reliable data and to

distinguish ultra thin coatings[1]

[1] Egerton, T.A., and I.R. Tooley, J. Mater. Chem. 12, 1111 (2002).

17

Wavenumber, cm-1

900100011001200

IR a

bsor

banc

e, a

.u.

0.0

0.1

0.2

0.3

0.4

0.5co-oxidized ySi/Al/TiO2

Al/TiO2

Wavenumber, cm-1

900100011001200

IR a

bsor

banc

e, a

.u.

0.0

0.1

0.2

0.3

0.4

0.5co-oxidized ySi/Al/TiO2

2.5 wt%

Al/TiO2

Wavenumber, cm-1

900100011001200

IR a

bsor

banc

e, a

.u.

0.0

0.1

0.2

0.3

0.4

0.5co-oxidized ySi/Al/TiO2

5 wt%2.5 wt%

Al/TiO2

Wavenumber, cm-1

900100011001200

IR a

bsor

banc

e, a

.u.

0.0

0.1

0.2

0.3

0.4

0.5co-oxidized ySi/Al/TiO2

10 wt%

5 wt%2.5 wt%

Al/TiO2

Wavenumber, cm-1

900100011001200

IR a

bsor

banc

e, a

.u.

0.0

0.1

0.2

0.3

0.4

0.5co-oxidized ySi/Al/TiO2

15 wt%

10 wt%

5 wt%2.5 wt%

Al/TiO2

Wavenumber, cm-1

900100011001200

IR a

bsor

banc

e, a

.u.

0.0

0.1

0.2

0.3

0.4

0.5co-oxidized ySi/Al/TiO2

20 wt% SiO2

15 wt%

10 wt%

5 wt%2.5 wt%

Al/TiO2

FT-IR

Si-O-Ti[1]Si-O-Si[1]

[1] Larouche S., H. Szymanowski, J.E. Klemberg-Sapieha, L. Martinu, and S.C. Gujrathi, J. Vac. Sci. Techn. A 22, 1200 (2004).

Chemical Structureco-oxidized

High and increasingintensity of Si-O-Si (asymmetric) band is attributed to theformation of large separate SiO2domains[2].

[2] Martins, O., and R.M. Almeida, J. Sol-Gel Sci. Technol. 19, 651 (2000).

Si-O-Si[1]

18

Wavenumber, cm-1

900100011001200

IR a

bsor

banc

e, a

.u.

0.0

0.1

0.2

0.3

0.4

0.5

Al/TiO2

SiO2-coated Al/TiO2

Wavenumber, cm-1

900100011001200

IR a

bsor

banc

e, a

.u.

0.0

0.1

0.2

0.3

0.4

0.5

2.5 wt% Al/TiO2

SiO2-coated Al/TiO2

Wavenumber, cm-1

900100011001200

IR a

bsor

banc

e, a

.u.

0.0

0.1

0.2

0.3

0.4

0.5

5 wt%

2.5 wt% Al/TiO2

SiO2-coated Al/TiO2

Wavenumber, cm-1

900100011001200

IR a

bsor

banc

e, a

.u.

0.0

0.1

0.2

0.3

0.4

0.5

10 wt%

5 wt%

2.5 wt% Al/TiO2

SiO2-coated Al/TiO2

Wavenumber, cm-1

900100011001200

IR a

bsor

banc

e, a

.u.

0.0

0.1

0.2

0.3

0.4

0.5

15 wt%

10 wt%

5 wt%

2.5 wt% Al/TiO2

SiO2-coated Al/TiO2

Wavenumber, cm-1

900100011001200

IR a

bsor

banc

e, a

.u.

0.0

0.1

0.2

0.3

0.4

0.5

20 wt% SiO2

15 wt%

10 wt%

5 wt%

2.5 wt% Al/TiO2

SiO2-coated Al/TiO2

Chemical Structure

Si-O-Ti

Si-O-Si

Si-O-Si

Shift of the Si-O-Si 1100 cm-1 band and high intensity of 1225 cm-1 band attributed to Si-O-Si bond strain[1,2].

The bond strain isreduced as coatingthickness increases.

[1] Almeida, R.M., and C.G. Pantano, J. Appl. Phys. 68, 4225 (1990). [2] Almeida, R.M., T.A. Guiton, and C.G. Pantano, J. Non-Cryst. Solids, 121, 193 (1990).

19

pH

2 4 6 8 10

Zeta

pot

entia

l ζ, m

V

-60

-40

-20

0

20

40

60pure Al/TiO2

pure SiO2

co-oxidized Si/Al/TiO2

Zeta Potential - Effect of SiO2 ContentAqueous suspensions of product particles

Isoelectric point (IEP)

pH 7.7pH 1.7Presence of both Al and Ti hydroxidesurface groups[1]

[1] Morris, G.E., W.A. Skinner, P.G. Self, and R.S. Smart, Colloid Surf. A-Physiochem. Eng. Asp. 155, 27 (1999).

IEP pure TiO2: pH 5 - 7[1]

IEP pure Al2O3 : pH 9[1]

20

pH

2 4 6 8 10

Zeta

pot

entia

l ζ, m

V

-60

-40

-20

0

20

40

60pure Al/TiO2

pure SiO2

co-oxidized Si/Al/TiO2

pH

2 4 6 8 10

Zeta

pot

entia

l ζ, m

V

-60

-40

-20

0

20

40

60

pure SiO2

co-oxidized Si/Al/TiO2pure Al/TiO2

2.5 wt% SiO2

pH

2 4 6 8 10

Zeta

pot

entia

l ζ, m

V

-60

-40

-20

0

20

40

60

5 "pure SiO2

co-oxidized Si/Al/TiO2pure Al/TiO2

2.5 wt% SiO2

pH

2 4 6 8 10

Zeta

pot

entia

l ζ, m

V

-60

-40

-20

0

20

40

60

5 "10 "pure SiO2

co-oxidized Si/Al/TiO2pure Al/TiO2

2.5 wt% SiO2

pH

2 4 6 8 10

Zeta

pot

entia

l ζ, m

V

-60

-40

-20

0

20

40

60

5 "10 "15 "pure SiO2

co-oxidized Si/Al/TiO2pure Al/TiO2

2.5 wt% SiO2

pH

2 4 6 8 10

Zeta

pot

entia

l ζ, m

V

-60

-40

-20

0

20

40

60

5 "10 "15 "20 "pure SiO2

co-oxidized Si/Al/TiO2pure Al/TiO2

2.5 wt% SiO2

Zeta Potential - Effect of SiO2 Content

co-oxidized The IEP shifts towardslower pH as the SiO2content increases and large SiO2 domains areformed in agreementwith FT-IR spectra.

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SiO2-coated Al/TiO2

pH

2 4 6 8 10

Zeta

pot

entia

l ζ, m

V

-60

-40

-20

0

20

40

60

pureSiO2

SiO2-coated Al/TiO2

5 " 10 "15 "20 "

pure Al/TiO2

2.5 wt% SiO2

SiO2-coated Al/TiO2

pH

2 4 6 8 10

Zeta

pot

entia

l ζ, m

V

-60

-40

-20

0

20

40

60

pureSiO2

SiO2-coated Al/TiO2

5 " 10 "15 "20 "

pure Al/TiO2

2.5 wt% SiO2

SiO2-coated Al/TiO2

pH

2 4 6 8 10

Zeta

pot

entia

l ζ, m

V

-60

-40

-20

0

20

40

60

pureSiO2

SiO2-coated Al/TiO2

5 " 10 "15 "20 "

pure Al/TiO2

2.5 wt% SiO2

SiO2-coated Al/TiO2

pH

2 4 6 8 10

Zeta

pot

entia

l ζ, m

V

-60

-40

-20

0

20

40

60

pureSiO2

SiO2-coated Al/TiO2

5 " 10 "

15 "20 "

pure Al/TiO2

2.5 wt% SiO2

SiO2-coated Al/TiO2

pH

2 4 6 8 10

Zeta

pot

entia

l ζ, m

V

-60

-40

-20

0

20

40

60

pureSiO2

SiO2-coated Al/TiO2

5 " 10 "15 "

20 "

pure Al/TiO2

2.5 wt% SiO2

SiO2-coated Al/TiO2

pH

2 4 6 8 10

Zeta

pot

entia

l ζ, m

V

-60

-40

-20

0

20

40

60

pureSiO2

SiO2-coated Al/TiO2

5 " 10 "15 "20 "

pure Al/TiO2

2.5 wt% SiO2

Zeta Potential - Effect of SiO2 Content

The negative zeta-potential at all pH for ≥ 10 wt% SiO2 might beattributed to difference in silica surface chemistrycompared to particleswith large SiO2 domainsas seen byFT-IR.

improved dispersionstability[1]

[1] Siddiquey, I.A., T. Furusawa, M. Sato, K. Honda, and N. Suzuki, Dyes and Pigments 76 754 (2008).

Very thin/partial coatingsat 2.5 - 5 wt% SiO2.

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Temperature, °C

100 200 300 400 500

Ther

mal

con

duct

ivity

(TC

) sig

nal,

a.u.

pure Al/TiO2

wt% SiO2

0 5 10 15 20

Ther

mal

con

duct

ivity

(TC

) pe

ak te

mpe

ratu

re, °

C150

170

190

210

230

250

270

Isopropanol Chemisorption

Temperature, °C

100 200 300 400 500

Ther

mal

con

duct

ivity

(TC

) sig

nal,

a.u. pure SiO2

pure Al/TiO2

Temperature, °C

100 200 300 400 500

Ther

mal

con

duct

ivity

(TC

) sig

nal,

a.u. pure SiO2

5 wt% SiO2-coated Al/TiO2

pure Al/TiO2

Temperature, °C

100 200 300 400 500

Ther

mal

con

duct

ivity

(TC

) sig

nal,

a.u. pure SiO2

20 wt% SiO2-coated Al/TiO2

5 wt% SiO2-coated Al/TiO2

pure Al/TiO2

277 °C

[2] Liu Z., J. Tabora, and J. Davis, J. Cat. 149, 117 (1994).

No desorption from a pure SiO2 surface[2]

Surface-adsorbate interactions weaker on silica-containing oxides[2]

coating processco-oxidized

Chemisorption at 110 °C[1]

Isopropanol release monitored by the change in the gas thermal conductivity

[1] Kulkarni, D., and I.E. Wachs, Appl. Catal. A-Gen., 237, 121 (2002).

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Conclusions

A process was developed for in-situ coating of flame-made particles.

Smooth and homogeneous SiO2coatings 2 – 4 nm thick on TiO2nanoparticles were obtained.

SiO2-coated TiO2 particles exhibitedlimited photoactivity.

The extent of SiO2 surface coverage was determined electrophoreticallyand chemically by isopropanolchemisorption.

N2

Si prec.